a) Field of the Invention
The invention is directed to a Schiefspiegler telescope with three reflecting surfaces in which an optical imaging by means of the three reflecting surfaces takes place extra-axially with respect to their respective center mirror axis, wherein the mirror axes of all of the reflecting surfaces are arranged within a plane. The invention is applied for observing the earth from space, particularly for imaging radiometers.
b) Description of the Related Art
Reflector telescopes of conventional construction in a rotationally symmetric mirror configuration (e.g., the systems of Gregory, Cassegrain and Newton) which are standard above all for astronomical observation suffer from contrast-reducing central vignetting due to the secondary mirror located in the beam path and the holding braces which are usually provided for it.
Reflector telescopes without vignetting, also called Schiefspieglers, generally comprise two to four mirrors arranged at various inclinations and constructed in different forms (concave, convex, partially parabolic to hyperparabolic, and so on). The correction of image errors allows only low light intensities, particularly in double-reflectors, or other costly corrective measures are required (e.g., corrective lens or toroidally deformed reflecting surface, and so on).
Triple-Schiefspieglers go back to the anastigmatic double-reflectors by A. Kutter, who selected a concave primary mirror and a convex secondary mirror because imaging errors such as coma, astigmatism and spherical aberration have opposite signs and therefore partially compensate one another. Chromatic aberrations are eliminated by omitting refractive surfaces. According to Kutter, vignetting is prevented because only the beam path which is free from silhouetting guarantees imaging with the highest possible contrast and resolving capacity.
In order to achieve improved optical imaging characteristics, the quantity of optically active surfaces, that is, the quantity of mirrors, is increased in reflector telescopes.
To enable a compact construction in spite of this, DE 196 49 841 C2 describes a compact Schiefspiegler in which the optical surfaces are used repeatedly. However, asymmetric mirror systems which are tilted parallel to a plane are used, and the problem of increased expenditure on adjustment remains unsolved.
Further, WO 03/02485 A2 discloses a scanning sensor system with rotating telescope components in which two telescope parts are moved at different rotational speeds. The problem of adjusting the mirrors relative to one another is also noted in this reference.
Another Schiefspiegler is known from EP 0 019 447 A1 which describes an anastigmatically imaging three-mirror optics system comprising a primary mirror with an ellipsoidal surface, a secondary mirror with a hyperboloid surface, and a tertiary mirror with another ellipsoidal surface. All three mirrors are in a plane, but are arranged off-center and at an inclination relative to an optical axis determined therein and form an off-axis system with respect to the aperture and field imaging. While the adjustments of the mirrors and angular settings within a plane are limited, the overall arrangement is still costly as concerns minimizing imaging errors.